1. Field of the Invention
This invention relates to a motor adopting an improved mechanism, particularly, relates to a motor of which hub is formed from a metal plate through a press working.
2. Description of the Related Art
A motor is commonly installed in a disc driving unit that drives a disc such as a hard disc and an optical disc to rotate.
It has been strongly required for such a disc driving unit to be thinner in thickness and lower in price as a portable disc driving unit such as a portable disc drive and a portable music player has been popularized recently.
As a matter of course, such a requirement has been also directed toward a motor to be installed in such a disc driving unit.
It has been commonly applied for a conventional motor that a rotor hub of the motor was produced from a rod stock of aluminum through a cutting process. In this regard, there has been a limit to thinning in thickness and lowering in price. In this connection, a rotor hub is generically referred to as “hub” in some cases.
Accordingly, in order to contribute to the above-mentioned market demand, various technologies for producing a rotor hub from a thin metal plate through a press working have been proposed. Some examples of such a technology are disclosed in the Japanese publication of unexamined patent applications No. 2003-338102 and No. 2001-245461 that was filed by the same inventor as the present invention.
The motors disclosed in the Japanese publication of unexamined patent applications No. 2003-338102 and No. 2001-245461 are a so-called shaft rotating type motor. In such a shaft rotating type motor, a radial bearing section such as a sleeve that supports a shaft axially is fixed to a stator side.
Further, both the shaft, which is supported by the radial bearing section so as to be rotatable freely, and a rotor hub, which is fixed to an outer circumferential surface of the shaft, rotate as a rotor.
In the meantime, when the rotor hub that is formed by the press working is fixed to the outer circumferential surface of the shaft, it is essential for the rotor hub to be provided with some amounts of engaging length with respect to the shaft along an axis of revolution of the shaft so as to obtain prescribed fixing strength between the rotor hub and the shaft or a prescribed degree of orthogonality with respect to the shaft.
In order to achieve an enough amount of engaging length, a through hole having a protruding section in a cylindrical shape is formed in the center of the rotor hub so as to engage the through hole of the rotor hub with the outer circumferential surface of the shaft.
Further, a bearing or an outer peripheral section of a sleeve is engaged with the through hole of the rotor hub.
More specifically, in the case of the Japanese publication of unexamined patent applications No. 2003-338102, a projected holding part 1c (see Abstract, FIG. 3 and paragraph [0019] in column 5) is equivalent to the above-mentioned protruding section of the rotor hub.
FIG. 7 is a cross sectional view of a conventional motor according to the prior art or the Japanese publication of unexamined patent applications No. 2001-245461.
Further, in the case of the Japanese publication of unexamined patent applications No. 2001-245461, as shown in FIG. 7, a holding section 106 in a cylindrical shape, which is formed on a rotor hub 105 and engages with a shaft 102, is equivalent to the above-mentioned protruding section of the rotor hub.
In such a structure of the rotor hub in which the protruding section of the rotor hub engages with the outer circumferential surface of the shaft, increasing an engaging length between the protruding section and the shaft is one method for improving the engaging strength and for obtaining the degree of orthogonality with a high degree of accuracy. However, the structure of the rotor hub creates a problem such that the motor results in increasing in total thickness.
On the contrary, in case of increasing the engaging length between the protruding section and the shaft without increasing total thickness of the motor, an engaging length between a radial bearing and a shaft is obliged to be shortened, and resulting in generating another problem such that the radial bearing fails to exhibit its primary performance and life of the bearing is shortened.
In other words, the other problem deteriorates dynamic characteristics of the motor and results in shortening the life of the motor.
Further, the rotor hub is provided with a flat surface on which a disc is mounted directly or through a member such as a sheet (hereinafter the “flat surface” is referred to as “disc mounting surface”). A location of the disc mounting surface in an axial direction affects a position of a disc surface directly, so that the location of the disc mounting surface must be positioned with a high degree of accuracy.
With respect to the disc mounting surface, in the Japanese publication of unexamined patent applications No. 2003-338102, a disk receiver 1b is exhibited as the disc mounting surface. In the case of the Japanese publication of unexamined patent applications No. 2001-245461, a flat surface 107 is shown as the disc mounting surface in FIG. 7.
However, the protruding section is formed by a burring process during the press working, so that an interval between a top surface of a flat section of the disc mounting surface and a tip of the protruded section is hardly obtained with a high degree of accuracy.
Accordingly, it is difficult to determine the location of the rotor hub in the axial direction in relation to the shaft with a high degree of accuracy.
Generally, in a disc driving unit, it is essential for an optical pickup or a head to be disposed without slanting with respect to a disc mounted thereon as far as possible. In this regard, it is desirable for a height or a location in the axial direction of the hub having the disc mounting surface to be determined with a high degree of accuracy.